Chemical polishing of metal surfaces



IT" Help y 1943- H. A. H. PRAY ETAL 2,446,060

" CHEMICAL POLISHING F METAL SURFACES Fi1edJuly4,l944

F/GURE 1 100% couc. NITRIC ACID A /o PHOSPHORIC ACID ACID AAAAAAAAAAAAAYAVAYAVAYAVAVAVAVAVAVAYA AVA so A AVAVAYAVAVAWAVAVAVAVAVAYA A 2VYYVVVVVVVVVVVVV 9o AAA AAAAAAAA HA AYAVAVAVAVAVAVAYAYAVAYAYAVAVA VA YYVVYVVVVYV V7 7 I007. CONC. IO 5O 6O 7O 8O I007. GLACIAL PHOSPHORIC AcloACETIC Aclo AGETIC ACID INVENTORS Henry A. Holden Pray Ivor lqolsrudGerald L. S imurd Mama;

AGE/VT Patented July 27, 1948 CHEMICAL POLISHING F METAL SURFACES HenryA. Holden Pray and Iver Igelsrud, Columbus, Ohio, and Gerald L. Simard,Stamford, Coma, assignors, by mesne assignments, to The BattelleDevelopment Corporation, Columbus, Ohio, a corporation of DelawareApplication July 4, 1944, Serial No. 543,450

8 Claims. 1

This invention relates to the polishing of metal surfaces. Moreparticularly it relates to a method and a composition for the polishingof such surfaces by chemical means, and it is primarily concerned withchemical treatments to polish the surfaces of metals and alloys such asGerman silver," coppe brass, nickel, and Monel."

, It has been common practice for many years to remove oxide scale frommetallic surfaces by various pickling treatments, and numerous types ofpickling baths have been developed. The object of such treatments is toremove the scale without, or with minimum, attack of the metal surface.More recently, a slightly different field of chemical treatment has beendeveloped in which the metal surface is attacked by the treatingsolution and brightened to a considerable degree. This latter treatment.usually termed "bright dipping," has been applied to numerous metals andalloys such as co er and ig alloys, aluminum and its alloyslmagnesiumanTits Alloys; silver and its alloys, nc, ca um, and

stainless steels.

In general, the baths most commonly used in the prior art for brightdippinghave been composed primarily of smiuriemcid, nijrio-aeld or ofmixtures of sulfuric and nitric acids. Minor additions of many othermaterials, such as other acids, metallic salts, organic compounds,carbonaceous materials, etc., have been suggested to aid in controllingthe character'of the brightening action of these baths.

For example, a recommended bright dipping procedure for German silver isas follows: First, treat the scale-free surface in a repicklin bathcomposed of liters of congeujg fif lliilm. 50 grams of sodiumhlbiide,and 0.2 liter of a so ution of soot in nitric acid; rinse; and treat n afinal brightening bath composed of 3 liters of concentrated nitric acid,5 liters of concentrated sulfuric acid, 40 grams sodium chloride, and0.2 liter'ofa'solution of soot in nitric acid.

A commonlyused bright clip for copper and its alloys consists of 2gallons of concentrated sulfuric acid, 1 gallon of concentrated nitricacid, A to ounce of sodium chloride, and 1 quart of water.

For nickel, the following procedure has been recommended: Dip for from 5to seconds in a bath composed of 1,000 cubic centimeters of water, 1,500cubic centimeters of concentrated sulfuric acid, 2,250 cubic centimetersof concentrated nitric acid, and 30 grams of sodium chloride; rinse; andneutralize in 1 percent ammonia solution.

The following brightening treatment has'been suggested for use inconnection with Monel: Dip in a bath composed of 1,000 cubic centimetersof water, 1,000 cubic centimeters of concentrated nitric acid, and 60 to90 grams of sodium chloride; rinse in hot water; dip for less than 5seconds in a bath composed of 1,000 cubic centimeters of water and 1,000cubic centimeters of concentrated nitric acid; rinse; and neutralize ina 1 percent ammonia solution.

A recommended bright dip for stainless steel comprises a solutioncontaining 25 per cent of concentrated hydrochloric acid, 5 per cent ofconcentrated nitric acid, and 0.5 per cent of a restrainer such as"Rodinefl all percentages by volume. This bath is preferably used atapproximately 150 F. There are, however, certain undesirable"f tures inthe .use of sulfuric and nitric acids,' either individua y or as acombination of the two, as righ p ng baths. In general, either suchbaths tend to be extremely reactive, in which case the work must beremoved quickly after insertion with resulting lack of control and highdrag-out losses, or they tend to be relatively inactive, in which caseexcessively long treatment times are required. Although the rapidreaction rates of some of the bright dips can be decreased considerablyby the addition of oxidizing acids such as chromlc, such additions arenot completely effective in solving the problem because, in these baths,they tend to decrease the uniformity of the etch. Furthermore, nitricacid alone or mixtures of sulfuric and nitric acids tend, during use asbright dips, to give off fumes of nitrogen dioxide. These fumes are notonly particularly corrosive, but they also have a marked physiologicaleffect and constitute an industrial hazard. Chromic acid additions havebeen found to be at least par- I tially effective in reducing thistendency. In addition, these prior art atbsahaileaaamrther drawback iresults from the attack of certain faces of the a metallic crystals,thereby producing facets which act as minute reflecting surfaces andcreate the brilliant appearance. Because of this etchin action, however,the articles so treated, even though brilliant, are not highlyreflective or mirror-like.

A still more recent development in the treatment of metal surfaces isthe discovery of anodic polishing. By this process, usually calledelectropolishing, which involves making the surface to be polished theanode in an. acid electrolyte, a smooth, plane surface is obtained. Thissurface is not only brilliant, as is the surface obtained by certainbright dips, but it is also unetched and, therefore, has the highlyreflective qualities of a mirror.

It is therefore one object of this invention to 'develop a chemicaltreatment for metal surfaces capable of producing a brilliant, smooth,highly reflective, unetched, polished surface.

Another object of our invention is to develop a chemical polishing bathfor the treatment of metal surfaces which has an intermediate reactionspeed, thereby making possible the proper control of the treatingoperation and permitting the polished articles to be drained for asufficient -length of time to prevent excessive bath losses fromdrag-out without requiring the use of excessive and uneconomically longtreating times.

A further object of our invention is to develop a bath for chemicallypolishing metal surfaces which does not give off appreciable quantitiesof corrosive or noxious gases.

Another object of our invention is to develop a bath for the chemicalpolishing of metal surfaces that has a solvent action for grease and oiland, therefore, does not reguire that all traces of grease and oil filmsBe' removed from the metal surface prior to treatment.

-Yet another object of our invention is to provide a bath for thepolishing of metal surfaces that is not extremely critical in regard tocomposition or to temperature of operation.

Still a further object of our invention is to develop a bath for theproduction of smooth, brilliant polishes on metal surfaces which has auniform action over large areas and which does not etch the surface eventhough the action is allowed to continue longer than is necessary forthe production of a polish.

Othe and further objects will be apparent from the following descriptionand appended claims.

In contrast to the bright etches resulting from the use of nitric acidor of mixtures of nitric and sulfuric acids, we have found that mixtures'of nitric acid plus acetic acid and phosphoric acid, either singly ormy in combination, 53c capable of producing lustrous, highly reflective,mirror-like surfaces on German silver, copper, brass, nickel, and Monel.Our invention, in its preferred form, is illustrated in the drawing andis hereinafter fully described.

In the drawing, the figure represents a triaxial diagram showing therelative proportions of riitric acid, acetic acid, and phosphoric acidapplicable for use in the polishing baths coming within The bathcompositions that are operative for the chemical polishing of theabove-noted metals and alloys are approximately defined ontheaccompanying triaxial diagram by the solid lines AB, BC, CD, DE, EF,FG, and GA, the approximate coordinates of the points being: A-5 percent concentrated nitric acid, 0 per cen acetic acid, and concentratedphosphoric acid; 3-75 per cent concen ra acid, 0 per cent glacial aceticacid, and 25 per cent concentrated phosphoric acid; C- per centconcentrated nitric acid, 15 per cent glacial acetic acid, and 10 percent concentrated phosphoric acid; D- per cent concentrated nitric acid,15 per cent glacial acetic acid, and 0 per cent concentrated phosphoricacid; E-35 per cent concentrated nitric acid, 65 per cent glacial aceticacid, and 0 per cent concentrated phosphoric acid; F-30 per centconcentrated nitric acid, 65 per cent glacial acetic acid, and 5 percent concentrated phosphoric acid; and G-5 per cent concentrated nitricacid, per cent glacial acetic acid, and 5 per cent concentratedphosphoric acid. Within the area so defined, any composition selectedwill be found to be operative in the method hereinafter described forthe chemical polishing of at least one of the metals selected from thegroup consisting of German silver, copper, brass, nickel, and Monel.

Although superior polishing is obtained within the range abovedescribed, in order to secure the very best results and the widestapplicability in polishing. with the most desirable reaction rates andthe greatest uniformity in operation, we prefer to operate withinsomewhat narrower limits. We have found that the range of proportions ofnitric acid, acetic acid, and phosphoric acid may advantageously be keptwithin the narrower limits represented on the accompanying diagram bythe area approximately defined by the dotted lines HI, IJ, JK, and KH,the approximate coordinates of the points being: H-iO per centconcentrated nitric acid, 10 per cent glacial acetic acid, and 80 percent concentrated phosphoric acid; 1-50 per cent concentrated nitricacid, 10 per cent glacial acetic acid, and 40 per cent concentratedphosphoric acid; J-50 per cent concentrated nitric acid, 40 per centglacial acetic acid, and 10 per cent concentrated phosphoric acid, andK-10 er cent concentrated nitric acid, 80 per cent glacial acetic acid,and 10 per cent concentrated phosphoric acid. The preferred compositionsof the baths for the chemical polishing of the hereinabove-referred tometals and alloys, with respect to the relative proportions of nitricacid, acetic acid, and phosphoric acid, lie within this second lesserarea which is wholly enclosed within the broader area first defined.

By German silver" we refer to the well-known type of nickel brass havinga composition within the following approximate range in percentages byweight: copper, 50 to 70 per cent; zinc, 5 to 30 per cent; and nickel,'7 to 30 per cent. In speaking of copper, we refer not only torelatively pure copper, but also to copper that may contain minoramounts of such elements as silver, arsenic, antimony, lead, selenium,cadmium, etc. The term brass is intended to cover copper-zinc alloys ofvarying compositions containing, in general, from 55 to per cent copperand from .5 to 45 percent zinc, by weight, some of which may containsmall percentages of other metals such as lead and tin. The nickelreferred to may be relatively pure nickel or it may contain minoramounts of such elements as cobalt, copper, iron, carbon and manganese.The term Monel refers to the well-known alloys containing, approx- 7 percent iron, and from 20 to 44 er cent copper,

by'weight. as well as up to several per cent of other minor elementssuch as aluminum, manganese, etc.

As is shown by the diagram, the composition of our polishing baths hassufi'icient latitude to en- 5 able commercial operation withoutprohibitively stringent control. In general, considering only therelative proportions of the three primary componentsconcentrated nitricacid, glacial acetic acid, and concentrated phosphoric acid, the broad lcomposition range may vary from about to 85 per cent concentrated nitricacid, from 0 to about 90 per cent glacial acetic acid, and from 0 toabout 95 per cent concentrated phosphoric acid.

Under the same conditions, the preferred com- 5 position ranges fromabout to 50 per cent concentrated nitric acid, from about 10 to 80 percent glacial acetic acid. and from about 10 to 80 per cent concentratedphosphoric acid.

Although the accompanying trlaxial diagram shows the relativeproportions of nitric acid, acetic acid, and phosphoric acid in a systemconsisting only of these three components, additional materials may bepresent. For example, a bath containing a ratio of nitric acid plusacetig acid and/or phosphorig acid sigected frorri't l igdiagram may, ifdesired, be dilute "withapgtain amount of watch, The maximum-ani'fimt ofwater that may be added without markedly impairing the character Lth parge extent upon'ili ratio of nitric, acetic, and phosphoric acidsselected. For example, baths composed of nitric and acetic acids, thosebaths lying on the line DE of the diagram, may be diluted with as muchas per cent of water; however, if this 3 4 On the other hand, bathscomposed of nitric and phosphoric acids, those'baths lying along the linAB of the diagram, show a noticeable loss in the brightness of theresulting polish when much more than 10 per cent of water is added tothe bath; and, in these baths, we prefer to keep the water added as suchbelow W cent of the total bath volume in order to obtain the very bestresults WifhTfii iturmfthe three acids, the maximum desirable wateradditions will lie somewhere between the above-disclosed limits. Inaddition, the baths also may include minor amounts of other substancessuch as ofi'alfii? compounds or other acids or salts, For example,

chromium trioxide or sulfamic acid may be add.-

e o cer a n o e ba 0 minimize gassing and fuming. Such additions areparticularly efficacious in the nitric acid-phosphoric acid combinationsor in the baths containing relatively large amounts of nitric acid andrelatively low amounts of acetic acid. We have found that acetic acid,in itself. is effective in reducing the fuming tendency of nitric acid;therefore, there is no particular advantage in adding chromium trioxideor sulfamic acid to baths containing api5 preciable amounts of aceticacid. In general, additions of up to about 10 per cent, based on thetotal weight of. the bath, of cITromiumt'rioxi'de or of sulfam c a areeffectivein minimizing fuming tendencies, For most cases. however, weprefer to add from about 0.5 to about 5 per cent of these Smalladditions of a wetting agent are efiective 'H'materials.

giin materially improving the character of the poltish obtained from thebath. In general, additions in the general range of from 0.05 to 1.0 percent. based on the total bath weight. are eil'ective for this purpose.

Also, the addition of traces of a chloride, such as may be added by theintroduction of small amounts of hydrochloric acid, sodium chloride,etc., materially increases the brightness of the resulting polish. Ingeneral, an addition chemically equivalent to that obtained from anaddition oi from 0.01 to 1.5 per cent of hydrochloric acid is sufficientfor this 5 urpose. In some cases. the addition of minor amounts ofvarious metal salts such as uranium acetate and cadmium or magnesiumnitrate gives slightly improved results.

Regardless of what other nonessential ingredients may be present in thebath, however, the relative proportions that the nitric acid, aceticacid, and phosphoric acid should bear to each other ma be obtained fromthe accompanying triaxial diagram.

Unless otherwise specified. all percentages referred to herein will beunderstood to represent percentages by volume rather than by weight.Also, the terms nitric acid, acetic acid}? and phosphoricacid refer tothe respective commercial strength concentrated acids. In other words.nitric acid" refers to commercial nitric acid having a specific gravityof 1.42, "acetic acid" refers to commercial glacial acetic acid having aspecific gravity of 1.05, and "phosphoric acid refers to commercialphosphoric acid having a specific gravity of 1.7.

It will be understood, of course, that, within the limits of additionalwater referred to above, diluted acids may be used. Also, althoughorthophosphoric acid is specifically referred to, chemically equivalentamounts of other forms of this acid, such as metaphosphoric andpyrophosphoric,

0 may be used. The chromium trioxide may be added as the compound CIO3,or chemicall equivalent additions of the various chromates or.dichroillates inay be substituted. Ifitlie"sfame bWTTFheEaTtEpIniramifcjiiani be substituted for the acid. In respect to the additionof wetting agents, referred to above, we have found that such agents asNacconol NR, a sodium alkyl aryl sulfonate; Duponol ME, an alkylsulfate, andFixanol', E'duaternary mn onium salt, and Orvus,sodiumlauryl sulfate, are qulte satisfactory. In cases wherea'ii'addition of water is referred to, the stipulated addition in excessof that normally present as a result of the use of commercial acids ismeant.

,As disclosed in the accompanying diagram and discussed hereinbefore,the polishing baths of this invention are composed of nitric acid pluseither acetic or phosphoric acids or a combination of the two. Ingeneral, the baths must contain at least approximately 5 per cent ofnitric acid or satisfactory polishes are not obtained. Baths containingless than about 5 per cent nitric acid tend either to be inactive or toetch the metal surface.

littleor no phosphoric acid, produce quite satisfactory polishes onGerman ilver and fair polishes on brass; however, approximately or 10per cent of phosphoric acid is required before the baths attain wideapplicability. As mentioned hereinbefore, superior results are obtainedwith the nitric acid-phosphoric acidmixtures when either chromiumtrioxide or sulfamic acid is present. In the polishing of nickel orMonel, it is desirable to have at least a trace of a chloride, such asis obtained from the addition of'a minor amount of hydrochloric acid,present; this is especially true in the case of nickel, as this metalshows a tendency to become passive unless a small amount of chloride ispresent.

When chromium trioxide, sulfamic acid, hydrochloric acid, or additionalwater is added to the polishing baths of this invention in an amountmuch exceeding .the hereinbefore disclosed limits, there is a tendencyfor the baths to stain and to produce a smooth etch and, therefore, togive surfaces comparable with the bright dips of the prior art ratherthan with the mirror-like, reflecting polishes obtained by means of ourinvention.

In connection with the foregoing discussion of the effect of theconcentration of the various components on the character of the polish,it will be understood of course that no sharp transition takes place inthe nature of the bath at the indicated concentration limits. Theselimits merely define what may be termed a threshold zone, and theexcellence of the polish and the flexibility of bath operation graduallydecrease as these limits are approached and crossed.

In the method of chemically polishing German silver, copper, brass,nickel, or Monel, using a bath of a composition indicated as suitable byreference to the accompanying triaxial diagram, the specified metal oralloy, or an article having a surface of the specified metal or alloy,is merely immersed in a bath of the selected composition for asufficient length of time to produce the desired high degree of polishon the metal surface.

. The time required to produce the desired polish will depend, ofcourse, upon the character of the original surface and upon thecomposition and the temperature of the bath. In general, polishing timeswill vary from seconds to 10 minutes; however, as will be hereinafterdiscussed, much longer times may be used without decreasing theexcellence of the polish. The bath temperature should preferably be keptbelow approximately 300 R, inasmuch as at higher temperatures,especially with baths containing acetic acid and phosphoric acid in thelower composition range, there is a tendency for excessive fuming totake place. Some of the disclosed baths, in fact, boil at temperaturesbelow 300 F., and in no case should the boiling temperature be exceeded.However, for most of the baths, we prefer to operate at temperaturesbetween room temperature, which may be considered to averageapproximately 65 F., and about 190 F. In fact, since only comparativelyshort periods of immersion, 15 seconds to several minutes, are requiredfor polishing in many of the compositions at room temperature, there islittle, if anything, to be gained by heating these baths. v

It is not necessary to agitate the work when polishing in the hereindisclosed baths. However, under certain conditions, especially in bathshaving nitric acid concentrations in the higher portion of thecomposition range, there may be some tendency for the polished surfaceto develop fur- 8. rows. Moderate agitation of the work is quiteeffective in eliminating such defects.

The polishing baths of this invention that contain appreciablequantities of acetic acid have a distinct advantage in that they are notextremely sensitive to the presence of oil and rease films on thesurface to be polished. Therefore, only moderate care is required in thepreparation of the surface prior to polishing in these baths. Ingeneral, it is desirable to remove any heavy scale by a normal picklingoperation and, if the surface is excessively oily or greasy, to degreaseit by any of the well-known methods, such as the alkaline cleaner. Withthis simple treatment, the surface is ready for polishing as disclosedherein. Our process may also be used to polish a surface that haspreviously been given a "bright dip" treatment.

The following examples are given by way of iilustration to show theoperation of our chemical polishing process in connection with thedisclosed metals and alloys. In describing the various finishesobtained, the following descriptive standards are used:

1. A polish-A highly reflective finish giving minimum distortion of theimage of a distant object.

2. 3" polish-A reflective finish giving recognizable but faintly fuzzyimages of objects more than an inch away. Perceptible graininess.

3. C polish-A reflective finish giving considerable fuzziness in theimages of objects more than one inch away. Readily visible graininess.

4. D polish-A reflective finish which shows a blurred image. Bright butgrainy. i

5. E finish-A semi-reflective finish which shows a blurred image and isgrainy but brightly and finely etched.

6. F etch'A slightly reflective surface which is brightly and smoothlyetched. More etched than grainy.

7. G etch-A bright, non-reflective finish with afine, smooth etch. Thissurface is typical of that produced by the bright dips of the prior 9.

8; H etch-A non-reflective finish with an etch which is not smooth.

Example 1 A bath containg 50 per cent nii'gic acid and 50 per centacetic acid was prepared. At a temperature of 70 F., this bath produceda B polish on German silver, containing per cent copper, 25

per cent zinc, and 10 per cent nickel, by weight, in approximately 30seconds. The sheet of German silver was then returned to the bath andallowed to remain for 25 minutes. At the end of this time, the sheet hadbeen dissolved until only a thin foil remained; there was, however, noetching, and the surface remained in the category of duced in about '30seconds on a sheet of brass 0011- taming approximately 35 per cent zinc,by weight. i

Example 2 Another bath, containing 20 per cent acetic acid and 80 percent nitric ,acid, was prepared. A sheet of German silver, containingapproximately '72 per cent copper, per cent zinc, and 18 per centnickel, by weight, was treated for one minute in this bath at atemperature of 80 F. A C polish was obtained, but there was sometendency for the surface to develop furrows. The specimen was re-treatedfor an additional minute with mild agitation. All of the tendency tofurrow was eliminated. Approximately 0.1 per cent, by weight, ofNacconol NR was added to the bath, and the specimen was again immersedfor approximately one minute. A B polish was produced.

Example 3 Another sheet of the German silver used in Example 2 wasimmersed in a bath containing 75 per cent acetic acid and 25 per centnitric acid. The bath temperature was 70 F. After one minute, thesurface had an F" etch. The treatment was continued for an additionalfive minutes. At the end of this time, the surface had an "H" etch andwas stained.

Example 4 A sheet of German silver containing 65 per cent copper, 20 percent zinc, and 15 per cent nickel, by weight, was immersed in a bathcontaining per cent acetic acid, 60 per cent nitric acid, and 15 percent water. With the bath at a temperature of 130 F., a 0" polish wasproduced in one minute.

Example 5 A bath containing 50 per cent ayeticacid and 50 per centphosphoric acid was prepared. This bath produced no action on Germansilver, copper, brass, nickel or Monel.

Approximately 5 per cent of nitric acid was added to the bath, makingthe composition approximately 5 per cent nitric acid, 47.5 per centacetic acid and 47.5 per cent phosphoric acid. This bath produced a Cpolish on the German silver containing 18 per cent nickel, by weight, inone minute at about 140 F.

Approximately 0.5 per cent of hydrochloric acid was added to this bath.With the bath temperature at about 140 F. and with a treatment time ofabout one minute, a D polish was produced on copper, and a similarpolish was produced on a brass containing about 15 per cent zinc,by'weight. Under these conditions, a "0 polish was produced on a sheetof Monel containing approximately 60 per cent nickel and 40 per centcopper, by weight.

Example 6 A bath containing 10 per cent nitric. acid and 90 per centphosphoric acid was prepared. About per cent, by weight, of chromiumtrioxide was added as sodium dichromateT'With the bath at roomtemperature, a surface characterized as a D polish was produced in 5minutes on a sheet of German silver containing 15 per cent nickel, byweight. Approximately 0.5 per cent, by weight, of Orvus was added to thebath and the temperature was raised to 180 F. In this bath, a similarsheet of German silver was polished to a C polish in two minutes. Thissheet was then returned to the bath and allowed to remain for minutes.At the end of this time, the sheet had been dissolved until only a thinfilm remained; there was, however, no etching,

10 and the surface remained in the category of a C polish,

At a bath temperature of 200 F., a "C" polish was produced on a sheet ofcopper, with a treating time of 2 minutes.

A sample of yellow brass, containing approximately 35 per cent zinc, byweight, was treated for 3 minutes in this bath at a temperature of 195F. A C polish was produced. This sheet was then returned to thepolishing solution for 45 minutes. At the end of this time, only a thinfoil remained. but the surface was still characterized as a- C polish.

The bath temperature was raised to 265 F.. and a sheet of nickel wastreated for 3 minutes. A "D polish was obtained. Approximately 1.0 percent, by weight, of sodium chloride was added, and a "C" polish wasproduced on a similar sheet in the same length of time.

A sheet of Monel, containing approximately 67 per cent nickel, 30 percent copper, and 1.4 per cent iron, by weight, was treated in this bathfor 3 minutes at a temperature of 215 F. A "C" polish was produced.

Example 7 A bath having the following composition was prepared: 75 percent nitric acid and 25 per cent phgsplgrigacid A sheet or German silverconfining 18 per cent nickel, by weight, was treated in this bath for 3minutes at a temperature of 160 F. A surface characterized as a D polishwas produced. The bath showed a pronounced tendency toward fuming.Approximately 3 per cent, by weight, of sulfamic acid and 0.3 per cent,by weight, of Fixanol were added to the bath. At a temperature of 110F., a "(1 polish was produced in approximately 5 minutes on anothersample of the German silver and on a sample of yellow brass similar tothat used in Example 6. The addition of sulfamic acid was increased to 6per cent, by weight, but the character of the polish obtained wasrelatively unchanged. Additions of either 3 or 6 per cent of thesulfamic acid were effective in reducing the fuming tendency of thebath.

Example 8 German silver, containing 10 per cent nickel, by weight, andred brass, containing 15 per cent zinc, by weight, were treated forapproximately 1 .minute in a bath containing 50 per cent nitric agigland 50 per cent phosphoric acid. With a bath temperature of 130 "F., a"0 polish was produced on both materials.

Example 9 A bath containing per cent nitric acid, 10 per centacetjcacid, and 10 per cent phosm qric acid, at a'temperature of F.,showed a marked tendency to produce an H etch on the surface of Germansilver, copper, brass, nickel and Monel. The bath action was veryviolent.

Example 10 taining approximately 70 per cent nickel and 30 per centcopper, by weight.

11 Approximately 0.01 per cent of hydrochloric acid was added to thebath, and the temperature was raised to 130 F. Under these conditions, aC polish was produced on German silver, brass and Monel. Copper obtainedan H etch and an E finish was formed on nickel.

Example 11 A bath composed of 70 per cent nitric acid, 15 per centacetic acid, and 15 per cent phosphoric acid, with a treating time of 30seconds at a temperature of 90 F., produced a C" polish on German silverand on yellow brass, a D" polish on Monel, and an H etch on copper andnickel.

Approximately 0.25 per cent of hydrochloric acid was added to this bathand the temperature was raised to 120 F. With a polishing time of 30seconds, a B polish was produced on German silver, a "C polish on brass,and a "D polish on copper and Monel. Nickel was given an H etch.

Example 12 A bath containing 65 per cent nitric acid, per

cent acetic acid, and 30 per cent phosphoric acid was prepared. At 90F., with a polishing time of 30 seconds, a C polish was produced onGerman silver containing per cent nickel, by weight. Under similarconditions, a "D polish was produced on red brass containing per centzinc, by weight.

About 1.0 per cent, by weight, of sodium chloride and 5 per cent ofwater were added to the bath, and the temperature was raised toapproximately 125 F. Under these conditions, a treatment for 30 secondsproduced a B' polish on the German silver, 2. "C" polish on copper andon the red brass, and a "D" polish on Monel containing 60 per centnickel and 40 per cent copper, by weight. Nickel was given an H etch.

Example 13 A polishing bath containing 39 per cent nitric acid, per centof acetic acid, and 55 per cent of phosp oric acid was prepared. About0.1 per cent, by weight, of Dupongllym and 0.5 per cent of hydrochloricacid"were added. At room temperature, and with a treating time of 4minutes, this bath produced a B polish on German silver containing 15per cent nickel, by weight, a "0 polish on copper; a "B polish on brasscontaining 35 per cent zinc, by weight, a D polish on nickel; and a Dpolish on Monel containing 67 per cent nickel, 30 per cent copper, and1.4 per cent iron, by weight.

The bath temperature was raised to 190 F. and the polishing time wasincreased to 5 minutes. Under these conditions, a B polish was producedon the German silver, a. "B" polish on copper, a C polish on the brass,and a C polish on the nickel and the Monel.

Example 14 Approximately 0.25 per cent, by weight, of Naccpnol HR wasadded to a bath containing 2.0 per cent nitric acid, 40 per cent aceticacid, 39.5 per cent phosphoric acid, and 0.01 per cent hydrochloricacid. At 190 F., a 0" polish was produced on German silver containing 10per cent nickel, by weight; a B polish was produced on copper; a Bpolish was produced on red brass; and a C polish was produced on nickeland on Monel containing 60 per cent nickel and 40 per cent copper, byweight.

Each of the metals was treated in the b h until only a thin foilremained. In no case was the character of the polish materially changed.

Example 15 A polishing bath composed of 40 per cent nitric acid, 30 percent acetic acid, and 30 per cent phosphoric acid was prepared; to thisbath was added 0.25 per cent of Orvus and 1.0 per cent of sodiumchloride, both by weight. Upon treatment for 4 minutes at F., this bathproduced a "B polish on German silver containing 15 per cent nickel, byweight, and C polishes on-copper, red brass, nickel, and Monelcontaining 70 per cent nickel and 30 per cent copper, by weight.

Approximately 15 per cent of water was added to the bath and theabove-outlined tests repeated. The character of the polishes wasunchanged.

Example 16 A sheet of German silver containing 15 per cent nickel, byweight, was treated for 2 minutes at 150 F. in a bath composed of 50 percent nitric acid, 40 per cent acetic acid, and 10 per cent phosphoricacid. A B" polish was produced. A similar sheet of the German silver wascovered with alight film of oil and treated under the same conditions. AB polish was again obtained.

These examples are given merely to illustra the principles,compositions, and methods of our invention, and it will be understoodthat variations are possible within the scope of the appended claims.

From the foregoing description of our invention, it will be apparentthat we have provided a novel polishing bath and a method whereby thesame may be used to produce highly reflective, lustrous polishes onGerman silver, copper,

to brass, nickel, and Monel. The reflectivity oi the surfaces polishedby means of our bath and method is superior to that produced by means ofthe bright dips of the prior art, the surfaces produced by the latterhaving a rating approximating a "G" etch on the hereindisclosed scale.It will also be'apparent that the operation of our chemical polishingbaths is much more flexible than that of the bright dips, which areextremely critical to such factors as composition, temperature, andpolishing time. Furthermore, water, in addition to that present as aresult of the use of commercial strength acids, may be added. Thistolerance is of particular importance in that articles to be polished donot have to be thoroughly dried prior to insertion in the polishingbath.

As disclosed hereinbefore, sugami g aggl may be used inpiace of chromiumtrioxide; we therefore wish it to be understood that in claimsspeciiying chromium trioxide, we can substitute sulfamic acid in placeof the chromium trioxide, and we wish to cover this.

Having thus described our invention, what we claim is:

l. A bath for chemically polishing metal surfaces, which consistsessentially of a mixture of concentrated solutions of nitric acid ofabout 1.42 sp. gr., phosphoric acid of about 1.7 sp. gr., and aceticacid 01' 1.05 sp. gr., which are present in relative percentages byvolume lying within the area defined approximately in the accompanyingdiagram by the solid lines AB, BC, CD, DE, El FG, and GA.

2. A bath'ior chemically polishing metal suriaces, which consistsessentially of a mixture of 13' concentrated solutions of nitric acid ofabout 1.42 sp. gr., phosphoric acid of about 1.7 sp. gr., and aceticacid of 1.05 sp. gr., which are present in relative percentages byvolume lying within the area defined approximately in the accompanyingdiagram by the dotted lines HI, IJ, JK, and KH.

3. A bath for chemically polishing metal surfaces, which consistsessentially of water not to exceed and a mixture of nitric acid having aspecific gravity of about 1.42, phosphoric acid having a specificgravity of about 1.7, and acetic acid having a specific gravity of about1.05 in which the relative percentages by volume of these three acidslie within the area defined approximately inthe accompanying diagram bythe solid lines HI, IJ, JK, and KH.

4. A bath for chemically polishing metals consisting essentially ofnitric acid, acetic acid, phosphoric acid and water, in amounts varyingfrom about 10 to about 50 per cent commercial nitric acid having aspecificgravity of about 1.42,

a from about 10 to about 80 per cent glacial acetic acid having aspecific gravity of about 1.05, from about 10 to about 80 per centcommercial phosphoric acid having a specific gravity of about 1.7, andup to about 10 per cent water, said percentage all being by volume.

5. A bath for the chemical polishing of metal, which consistsessentially of nitric acid having a specific gravity of about 1.42,acetic acid having a specific gravity of about 1.05, phosphoric acidhaving a specific gravity of about 1.7, and water in an amount up toabout 10 per cent of the total volume of the bath, the above named acidsbeing present in the relative percentages by volume lying within thearea defined approximately in the accompanying diagram by the solidlines AB, BC, CD, DE, EF, PG, and GA.

0. The method of treating metal surfaces of the group consisting ofGerman silver, copper, brass, nickel, and Monel to produce brightpolishes thereon, which comprises immersing the surface to be polishedin a bath consisting essentially of a mixture of concentrated solutionsof nitric acid of about 1.42 sp. gr., phosphoric acid of about 1.7 sp.gr., and acetic acid or about 1.05 sp. gr., the relative proportions byvolume of these three ingredients lying within the area definedapproximately in the accompanying diagram by the solid lines AB, BC, CD,DE, EF, FG, and GA, at a temperature of less than 300 F. and

14 for a suflicient time to'efiect the desired degree of polish.

7. A method of producing a bright polish on metal surfaces of the groupconsisting of German silver, copper, brass, nickel, and Monel whichcomprises immersing the surface to be polished in a bath consistingessentially of a mixture of concentrated solutions of nitric acid ofabout 1.42 sp. gr., phosphoric acid of about 1.7 sp. gr., and aceticacid of about 1.05 sp. gr. present in the relative proportions by volumelying within the area defined approximately in the accompanying diagramby the dotted lines HI, IJ, JK, and KH, at a temperature of less thanabout F. and for a suflicient time to effect the desired degree ofpolish.

8. The method of producing a brilliant, reflective surface on a metalselected from the group consisting of German silver, copper, brass,nickel, and Monel, which comprises immersing the metal surface to bepolished in a bath consisting essentially of a mixture of concentratedsolutions of nitric acid of about 1.42 sp. gr., phosphoric acid of about1.7 sp. gr., and acetic acid of about 1.05 sp. gr. present in relativeproportions by volume lying within the area defined approximately in theaccompanying diagram by the dotted lines AB, BC, CD, DE, EF, PG, and GA,and containing up to about 10% water.

HENRY A. HOLDEN PRAY. IVER IGELSRUD. GERALD L. SIMARD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,816,837 Ganser Aug. 4, 19311,919,624 Dunn et al July 25, 1933 1,939,241 Taylor Dec. 12, 19332,070,487 Lutz Feb. 9, 1937 2,172,171 Meyer et al Sept. 5, 19392,238,778 Malcolm Apr. 15, 1941 FOREIGN PATENTS Number Country Date174,489 Great Britain Feb. 2, 1922 489,211 Great Britain July 21, 1938

